Method of combustion using high temperature stable catalysts

ABSTRACT

Catalysts having good high temperature stability which are particularly useful for selected high temperature reactions are disclosed as are methods for their preparation and use. The catalytically-active materials include platinum group metal deposited on a catalytic slip or composite which contains a mixture of alumina, a rare earth metal oxide, and a mixture of metallic oxides of selected IIA and IVA metals and selected VIB metals. The slips or carrier compositions are calcined at a temperature of at least 500°C. before deposition of platinum group metal and characterized by having a surface area of at least 20 m 2  /g after calcination at a temperature of 1200°C. for two hours.

The present invention relates to catalyst compositions and methods fortheir preparation and use. In particular, this invention relates tocatalyst composition characterized by high stability thereby maintaininggood catalytic activity.

Catalyst compositions exhibit a relatively high surface area per unitweight to allow the largest amount of reactants to contact the catalyst.Additionally, high surface area is important when the catalystcomposition contains a precious metal such as platinum because of thecost of the metal and because of the dispersion required to preventundue metal crystallite growth. It is desirable to retain this highsurface area for long periods of use under servere conditions whichmight include reaction temperatures of 1200°C. or higher.

Alumina is an excellent and relatively economical carrier or support formany catalysts. Many crystalline forms of alumina, for example, chi,kappa, gamma, delta, eta, and theta, exhibit a very high surface area inrelation to their weight. A serious drawback of alumina as a catalystcarrier, however, is its transition temperature of about 1000°-1200°C.to the alpha form which results in a substantial reduction of thesurface area. It is thus extremely desirable to stabilizealumina-containing catalyst compositions based on high surface areaaluminas to substantially prevent the transition to the low surfacealpha form with a consequent loss in activity.

It is therefore an object of this invention to provide catalystcompositions, as well as methods for their preparation and use, whichexhibit high temperature stability. Other objects and advantages willappear as the description proceeds.

Broadly, the catalyst composition of the invention includes acatalytically-active, calcined composite characterized by a surface areaof at least 20 square meters per gram (m² /g) after calcination for twohours at a temperature of 1200°C., said composite comprising or being acomposite of alumina, a rare earth metal oxide and a mixture of twometal oxide components wherein the metal of the first metal oxidecomponent is selected from the group consisting of Cr, W and mixturesthereof and the metal of the second oxide component is selected from thegroup consisting of Ca, Ba, Sr, Si, Sn, and mixtures thereof. Inpreparing the catalyst composition, the composite is first calcined at atemperature of at least 500°C. and then a catalyticallyeffective amountof a platinum group metal is added to the composite. A catalystcomposition prepared in accordance with this invention exhibits hightemperature stability and therefore catalytic activity in a number ofhigh temperature reactions, particularly high temperature combustionreactions.

The composite is formed by the calcination of an intimate admixture ofan aluminum compound, rare earth metal compound and a mixture of twometal oxide components wherein the metal of the first metal oxidecomponent is selected from the group consisting of Cr, W and mixturesthereof and the metal of the second oxide component is selected from thegroup consisting of Ca, Ba, Sr, Si, Sn, and mixtures thereof.Preferably, for certain methods of preparation, the aluminum compound isalumina. These compounds, as indicated, if not already in oxide formmust be capable of forming or yielding their respective oxides uponcalcination in air (oxygen) at a temperature of at least 500°C. Thecombination of the rare earth metal oxide and the other metal oxide oroxides may be considered as a high temperature stabilizing component forthe alumina.

The relative amounts of alumina to the metal oxide stabilizingcomponent, that is, the rare earth metal oxide and oxides of the metalsof the Group IVA and/or IIA metals and chromium and tungsten and/ormixtures of these compounds, are governed largely by empirical criteria.While it is not desired that this invention be limited by the followingtheory, a brief statement may provide a helpful framework to furtherelucidate the invention. It is thought that the addition of thestabilizing component to the alumina of alumina precursor andcalcination of the mixture at a temperature of at least 500°C. convertsany of the nonoxide compounds to oxides and allows the stabilizingcomponent oxides to enter the alumina lattice and prevent orsubstantially reduce subsequent transition to alpha alumina.

All surface areas throughout the specification and the appended claimsare measured by the B.E.T. or equivalent method. The terminology used todescribe the metals herein, that is, the rare earth or lanthanide seriesmetals is the terminology used in association with the common long formof the Periodic Table of Elements. Thus the rare earth or lanthanidemetals are metals of atomic number 57 to 71.

The catalyst composition may also contain a minor amount of otheringredients, up to about 5 percent by weight of the composite, which mayserve as promoters, activators, or other purposes, for oxidation orreduction reactions. Such ingredients may include, for example,manganese, vanadium, copper, iron, cobalt, and nickel usually as themetal oxide or sulfide.

The calcined composite may be formed to any desired shape such as apowder, beads, or pellets. This shaping or fabricating is accomplishedbefore calcination to promote particle adhesion. After calcination, aplatinum group metal is added to the composite. Additionally, thecomposite can be applied or deposited on a relatively inert support orsubstrate and the platinum group metal then added, or the catalystcomposition can be applied or deposited onto the inert support.

For compositions made in accordance with this invention, the compositegenerally comprises about 5 to 95 weight percent alumina, and about 2 to25 weight percent of rare earth metal oxide, preferably about 5 to 15weight percent, based on the total weight of composite. The Ca, Ba, Sr,Si, and Sn metal oxide may be present in about 2 to 15 weight percent ofthe composite, preferably about 5 to 15 weight percent. The chromiumand/or tungsten oxide may be present in about 2 to 15 weight percent,preferably about 5 to 15 weight percent of the composite. The mixturesof Ca, Ba, Sr. Si, and/or Sn metal oxides and chromium and/or tungstenoxides may be present in about 5 to 30 weight percent, preferably about5 to 15 weight percent of the composite. If the amount of alumina is toolow, the resulting composite will not provide enough surface area toprovide catalytic activity. If more alumina is present than stated, itmay not be stabilized sufficiently and will lose surface area in thetransition to the alpha form.

Generally, to provide the advantages of this invention, it is necessaryfor the stabilizing component to be in intimate association with thealumina during pre-calcining. An intimate admixture may be achieved, forexample, by forming a slurry of alumina with water soluble compounds ofthe stabilizing components. Where desired, hydrated alumina, such asaluminum trihydrate is admixed with aqueous solutions of a rare earthmetal salt and the mixture of the other metal salts of this invention topermit sorption of the stabilizing The by the alumina. The solids arethen recovered from the slurry and calcined to provide the mixed oxidecomposite. Theh particulate alumina is preferably in finely divided orcolloidal form to provide maximum sorption area. For example, finelydivided freshly precipitated aluminum trihydrate having a particle sizeof 70 percent to 90 percent smaller than 325 mesh is useful. When largeparticle size alumina is used, the sorption of the stabilizingcomponents from solution and subsequent calcination will provide atleast a stabilized outer portion of the alumina.

Another method of preparing intimate admixture of alumina andstabilizing components is to coprecipitate all of the components,including the alumina, from aqueous solutions. Various methods ofcoprecipitation are suitable. Such methods include, for example, surfaceadsorption where one or more components in ionic form are sorbed on thesurface of a precipitating solid; and inclusion, in which thecoprecipitated compound or compounds have dimensions and a chemicalcomposition which will fit into the crystal structure of a precipitatingsolid without causing appreciable distortion.

In coprecipitation, a suitable precipitant, usually a base, is added toan aqueous solution of the compounds. This can also be done byconcurrent addition of both the precipitant and the compound solution toa vessel containing water. Preferably the precipitant is selected suchthat undesirable or unnecessary compounds are volatilizable anddecomposable upon calcination at 500°C. or above, or removable bywashing or extraction. The precipitant is capable of initiating andcompleting essentially simultaneous coprecipitation of the components.Suitable precipitants are ammonium compounds such as ammonium hydroxideor ammonium carbonate as well as other hydroxides and carbonates of thealkali metals.

The precipitant may be in dilute or concentrated aqueous solution. Therapidity of addition of the precipitant and the degree of agitation usedwill vary depending upon the precipitate desired. Dilute precipitantsolutions, slow addition, and vigorous agitation generally favor acoarser precipitate. The temperature during the addition of precipitantmay be from about 0° to 90°C. Higher temperatures generally produce acoarser precipitate. The precipitant is added until a pH of about 5 to9.0 is reached. At this time the coprecipitated mixture is recoveredfrom the slurry, washed if desired, and digested or recrystallized ifdesired.

The intimate admixture of alumina and stabilizing components arecalcined at a temperature of at least about 500°C., preferably about900° to 1200°C., but not at such a high temperature or for such a longperiod of time to unduly sinter the composite. The conditions of thecalcination are such as to provide a catalytically-active compositehaving a relatively high surface area of at least about 25 square metersper gram, and preferably at least about 75. Calcination is preferablyconducted while the admixture is unsupported and in free-flowingcondition. This is preferable for economic reasons and to prevent unduesintering.

Calcination in air to form the composite, and prior to the addition of aplatinum group metal, is an integral part of the subject invention. Itis found that an intimate admixture of the stabilizing components andthe alumina is stable when calcined at such temperatures before anyfurther preparative steps are performed. Since both the alumina and thestabilizing components are intimately admixed, the concurrent heating inclose association substantially reduces any undesirable aluminatransitions. Additionally, calcination before deposit on an inertsubstrate promotes adhesion of the calcined composite to the substratethus allowing the use of higher space velocities with the finishedcatalyst composition with less chance of erosion. Further, calcinationsubstantially reduces the possibility of reaction of the stabilizingcomponent and alumina component with the substrate. Any such reactionsbetween the alumina and the substrate promotes the formation of inactiveforms of alumina thereby reducing its surface area and activity. If thestabilizing component were to react with the substrate, it would reducethe effective amount of this component available for stabilization. Afurther advantage of such calcination is economic because less heat insmaller furnaces is required to calcine the resulting powder compositebefore it is placed on an inert support. Further, it is essential thatthe calcination is conducted before the addition of a platinum groupmetal component to prevent loss of such component by occlusion.

Suitable aluminum-containing compounds are alumina, the gamma, eta,kappa, delta, and theta forms of alumina and for coprecipitation, thewater soluble aluminum compounds such as salts, for example, thealuminum halides, aluminum nitrate, aluminum acetate, and aluminumsulfate.

The rare earth metal compounds which may be employed to produce thecatalytic composite are, for example, the compounds of cerium,lanthanum, neodymium, samarium, praseodymium, and the like as well ascommercially available mixtures of rare earths. The rare earth used ispreferably cerium. If a mixture of rare earths is used, the mixture ispreferably one in which cerium is the predominant component. Suitablewater soluble rare earth metal compounds include the acetates, halides,nitrates, sulfates, and the like, e.g., Ce(C₂ H₃ O₂)₂, CeBr₃, Ce(NO₃)₃,Ce₂ (SO₄)₃, Nd(C₂ H₃ O₂)₃, Sm(NO₃)₃, and TmBr₃.

The oxides of calcium, barium, and strontium are added to the alumina inthe form of their water soluble precursors. Thus, for example, watersoluble metal salts such as the nitrates, acetates, selected halides,and the like might be employed. Suitable water soluble compounds areBa(NO₃)₂, Ca(NO₃)₂, Sr(NO₃)₂, and Ba(C₂ H₃ O₂)₂.

Water soluble compounds of chromium and tungsten which can be used are,for example, chromium acetate, chromium nitrate, chromium halides,chromium oxide (chromic acid), chromium oxalate, and complexes ofchromium such as chloropentamine chromium chloride, tungsten halides,tungsten oxy-salts, such as tungsten dioxydichloride, ammoniumtungstate, and the like.

Suitable Group IVA compounds are compounds of silicon and tin includingwater soluble salts and organic complexes as well as certain dispersiblecompounds. The halides and sulfates of tin are useful as well as certaindispersions of high surface area, low sodium, colloidal silica having avery small particle size, for example, a produce marketed by Du Pont deNemours & Co., under the registered Trademark Ludox LS is particularlysuitable. This silica sol contains about 30% by weight SiO₂ in water,has a particle size of about 15 millimicrons, a 285 SiO₂ to Na₂ O ratioand a surface area of about 200 m² /g.

A platinum group metal is added to the calcined composite to form thecatalyst compositions of this invention, which are found to be effectivefor long time high temperature reactions. Such metals are usually addedor incorporated in amounts sufficient to provide significant activity.The platinum group metals useful are platinum, ruthenium, palladium,iridium, and rhodium. The choice of metal, metal combinations or alloysis governed largely by activity, specificity, volatility, deactivationby specific components included with the reactants, and economics.

The quantity of platinum group metal added to the calcined compositedepends first on design requirements such as activity and life andsecond on economics. Theoretically, the maximum amount of such metal isenough to cover the maximum amount of surface available without causingundue metal crystallite growth and loss of activity during use. Twomajor competing phenomena are involved in such surface treatment. It isdesirable to completely cover the substrate surface to provide thegreatest amount of platinum group metal coverage, thereby obtainingmaximum activity, but if the surface were to be completely covered, suchcoverage would promote growth between adjacent crystallites, whichgrowth would then decrease the surface area and greatly reduce activity.A balance of maximum coverage coupled with proper dispersion thus mustbe achieved to formulate a practical catalyst. An ancillaryconsideration in relation to the amount of platinum group metal is theallowable size of the catalyst housing. If the size is small, the amountof platinum group metal component used is preferably increased withinthe above-described limits. For example, for automobile exhausttreatment, the allowable size is relatively small, especially if unitaryhoneycomb type supports are used and a higher loading may be desirable.Economics, of course, dictates the use of the least amount of platinumgroup metal component possible while accomplishing the main objective ofpromoting the reaction. Generally, the amount of platinum group metalused is a minor portion of the catalyst composite and typically does notexceed about 20 weight percent of the calcined composite. The amount maybe about 0.1 to 20 percent and is preferably about 0.2 to 10 percent toeconomically maintain good activity with prolonged use. Thesepercentages are based on the weight of the calcined composite. If thecomposite is used on the inert substrate, the composite may be, forexample, about 10 percent of the weight of the substrate and the percentweight of platinum group metal in relation to the total weight ofsubstrate and composite will be correspondingly less.

During preparation of the catalyst composition, various compounds and/orcomplexes as well as elemental dispersions of any of the platinum groupmetals may be used to achieve deposition of the metal on the composite.Water soluble platinum group metal compounds or complexes may be used.The platinum group metal may be precipitated from solution, for example,as a sulfide by contact with hydrogen sulfide. The only limitation onthe carrier liquids is that the liquids should not react with theplatinum group metal compound and be removable by volatilization ordecomposition upon subsequent heating and/or vacuum, which may beaccomplished as part of the preparation or in the use of the completedcatalyst composition. Suitable platinum group metal compounds are, forexample, chloroplatinic acid, potassium platinum chloride, ammoniumplatinum thiocyanate, platinum tetrammine hydroxide, platinum groupmetal chlorides, oxides, sulfides, and nitrates, platinum tetramminechloride, palladium tetrammine chloride, sodium palladium chloride,hexammine rhodium chloride, and hexammine iridium chloride. If a mixtureof platinum and palladium is desired, the platinum and palladium may bein water soluble form, for example, as ammine hydroxides or they may bepresent as chloroplatinic acid and palladium nitrate when used inpreparing the catalyst of the present invention. The platinum groupmetal may be present in the catalyst composition in elemental orcombined forms, e.g., as an oxide or sulfide. During subsequenttreatment such as by calcining or upon use, essentially all of theplatinum group metal is converted to the elemental form.

While these catalyst compositions are useful in many reactions, they arenot necessarily equivalent in all processes nor are those which areuseful in the same process necessarily exactly equivalent to each other.

While it is not essential, the catalyst compositions of this inventionpreferably have a relatively catalyticallyinert support or substrate.The supports which can be employed in this invention are preferablyunitary, skeletal structures of relatively large size, e.g., honeycombs.However, smaller particle forms may be used, e.g., pellets or spheres.The size of these pellets can be altered depending upon the system, itsdesign and operating parameters in which they are to be used, but mayrange from about 1/64 to 1/2 inch, preferably 1/32 to 1/4 inch, indiameter; and their lengths are about 1/64 to 1 inch, preferably about1/32 to 1/4 inch.

When a support is used, the calcined composite is generally present in aminor amount of the total catalyst composition, which is usually about 2to 30 weight percent preferably about 5 to 20 weight percent, based onthe total weight of the composite and support. The amount used dependson economics, size limitations, and design characteristics.

These supports whether of the unitary-skeletal type or pellets arepreferably constructed of a substantially inert, rigid material capableof maintaining its shape and strength at high temperatures, for example,up to about 1800°C. The support typically has a low thermal coefficientof expansion, good thermal shock resistance, and low thermalconductivity. While a support having a porous surface is preferred, thesurface may be relatively non-porous, but in such event it is desirableto roughen the surface to improve adhesion of deposited compositions.

The support may be metallic or ceramic in nature or a combinationthereof. The preferred supports, whether in skeletal or other form, arecomposed primarily of refractory metal oxide including combined oxideforms, e.g., aluminosilicates. Suitable support materials includecordierite, cordierite-alpha alumina, silicon nitride, silicon carbide,zircon-mullite, spodumene, alumina-silica-magnesia, and zirconiumsilicate. Examples of other suitable refractory ceramic materials aresillimanite, magnesium silicates, zircon, petalite, alpha-alumina, andaluminosilicates. Although the support may be a glass ceramic, it ispreferably unglazed and may be essentially entirely crystalline in formand marked by the absence of any significant amount of glassy oramorphous matrices. Further, the structure may have considerablyaccessible porosity, preferably having a water pore volume of at leastabout 10 percent. Such supports are described in U.S. Pat. No.3,565,830, herein incorporated by reference.

The geometric, superficial, or apparent surface area of the skeletal orhoneycomb type supports, including the walls of the gas flow channels isgenerally about 0.5 to 6, and preferably 1 to 5, square meters per literof support. This surface area is sufficient for deposition of asatisfactory quantity of the composite or the finished catalystcomposition. The plurality of channels, about 100 to 2500, preferably150 to 500 per square inch of crosssectional area, may be distributedacross the entire face of the structure and frequently they define anopen area in excess of 60 percent of the total area of the support. Thewalls must be thick enough to provide rigidity and integrity to thestructure while maintaining good apparent surface area. The wallthickness is thus in the range of about 2 to 25 mils. The flow channelscan be of any shape and size consistent with the desired superficialsurface area and should be large enough to permit relatively freepassage of the gaseous reaction mixture; preferably the length of thechannels is at least about 0.1 inch to insure sufficient contact orresidence time to cause the desired reaction. Although the channels aregenerally parallel, they may be multi-directional and may communicatewith one or more adjacent channels.

In one manner of preparing structures provided with catalystcompositions of this invention, an aqueous slurry of the essentiallywater insoluble calcined composite of alumina and stabilizing componentis contacted with the support. The solid content of the slurry forms anadherent deposit on the support, and the resulting supported compositeis dried or calcined for a second time at a temperature which provides arelatively catalytically-active product. The second drying orcalcination takes place at a temperature low enough to prevent unduesintering of the mixture. Suitable calcination temperatures aregenerally about 300°-700°C. to insure catalytic activity without unduesintering, preferably about 400°-600°C. After this second calcinationthe coating on the support has a surface area of at least about 75s.m.p.g. Lower temperatures can be employed to dry the composite if thesecond calcination is not performed.

After the coated support is dried or calcined, a platinum group metalcomponent is added to enhance the catalytic activity of the composite.The platinum group metal may be added to the coated support in themanner previously described. Preferably, this addition is made from anaqueous or other solution to impregnate or deposit the platinum groupmetal component on the coated support.

After addition of the platinum group metal, the resulting structure isdried and may be calcined for a third time under conditions whichprovide a composition having characteristics that enhance selectedreactions. This final calcination stabilizes the completed catalystcomposition so that during the initial stages of use, the activity ofthe catalyst is not materially altered. The temperature of this finalcalcination must be low enough to prevent substantial sintering of theunderlying coating which would cause substantial occlusion of theplatinum group metal component. Thus the calcination may be conducted attemperatures of about 300°-700°C., preferably about 400°-600°C.

An alternative method of making the catalyst compositions of thisinvention if a relatively inert support is used involves adding theplatinum group metal component to the calcined composite before thecomposite is deposited on the support. For example, an aqueous slurry ofthe calcined composite can be prepared and the platinum group metalcomponent added to the slurry and mixed intimately therewith. Theplatinum group metal component can be in the form already described andmay be precipitated as previously described. The final mixturecontaining the platinum group metal may then be dried or calcined toprovide a catalytically-active composition in a form suitable fordeposition on a support or for use without such deposition as a finishedcatalyst in either finely divided or macrosize forms. Subsequentcalcinations or drying may be conducted as described above. The calcinedmaterial generally has a surface area of at least about 25 s.m.p.g.,preferably at least about 75 s.m.p.g.

The following are examples of the general method of preparation of somerepresentative stabilized catalytic composites and compositions of thisinvention. All percentages, parts, and proportions herein and in theappended claims are by weight unless otherwise indicated.

EXAMPLE I

A stabilized Nd₂ O₃, Cr₂ O₃, SrO and Al₂ O₃ composite is prepared. 7.82grams of neodymium nitrate, 3.95 grams of CrO₃ (chromic acid) and 6.13grams strontium nitrate are dissolved in H₂ O to form a total volume of80.3 ml. 51 grams of activated Al₂ O₃ powder is stirred into thesolution with constant agitation for 10 minutes. The total solution isthen evaporated to dryness under heat and with agitation, transferred toa drying oven at 110°C., and dried overnight. The dried solids are thencrushed to powder. Five grams of the composite containing 5% neodymia,5% chromia, 5% strontia, and 85% alumina is then tested for retention ofsurface area by calcining at 1200°C. for 4 hours. It is found that thesurface area after such calcination is 24.0 m² /g.

EXAMPLE II

A two kilogram batch of composite is prepared exactly as in EXAMPLE I.The powder is then calcined at about 750°C. for one hour. 150 grams ofthe calcined powder thus prepared are mixed with 250 ml. H₂ O and 11 ml.conc. HNO₃, and ball-milled for 19 hours at 68 RPM in a U.S Stoneware1-gallon mill jar. 275 ml. of the resulting slip are diluted with waterto a viscosity of about 68 cps. A 20 cubic inch cordierite honeycombhaving about 250 parallel gas passages per square inch ofcross-sectional are is dipped into this diluted slip, drained, blownwith air, dried at 110°C. for 21/2 hours, and calcined at 500°C. for 2hours. The adherent composite is coated on the honeycomb.

EXAMPLE III

A honeycomb, coated with a neodymia-chromia-strontia-alumina compositeslip is prepared as in EXAMPLE I. The coated honeycomb is then dippedinto about 420 ml. of a solution containing both H₂ PtCl₆ and Na₂ PdCl₄,concentrations of each being such that there is theoretically 0.9% Pt byweight of solution and 0.3% Pd by weight of solution. After standing for10 minutes with intermittent raising and lowering of the honeycomb intothe solution, the honeycomb is withdrawn from the solution, drained, andexcess solution blown off. The honeycomb is then treated with gaseoushydrogen sulfide for 15 minutes, and washed chloride-free usingdeionized water. The resulting impregnated honeycomb is dried overnightat 110°C., and calcined in flowing air for 2 hours at 500°C. Thefinished catalyst contains both Pt and Pd.

EXAMPLE IV

A composite is prepared containing a commercial rare earth mixture,chromia, baria, and alumina. 25.57 grams of Ba(NO₃)₂ is dissolved inwater by warming to 55°C. This solution is then diluted to 400 ml and255 grams of commercial alumina having a surface area of about 300 m² /gis added to the barium solution. The slurry is mixed for about 5 minutesdried for 1 hour with heat, transferred to an oven and dried overnightat 110°C. 285 grams are recovered and this is crushed to powder. Thispowder (admixture A) is set aside for further preparations. 7.43 gramsof a mixture of rare earth nitrates is used to form the totalcomponents. The composition converted to the theoretical oxide contentis as follows: CeO₂ 48%; La₂ O₃ 24%; Nd₂ O₃ 17%; Pr₆ O₁₁ 5; Sm₂ O₃ 3%;Gd₂ O₃ 2%; Y₂ O₃ 0.2%; others 0.8%. The rare earth mixture and 3.95grams of CrO₃ are dissolved in water and diluted to 70 ml. 57 grams ofadmixture A, i.e., the alumina-barium admixture, is added to thesolution with agitation for 5 minutes. The slurry is transferred to anevaporating dish, dried with agitation for one hour under an infraredlamp, transferred to an oven and dried at 110°C. overnight. The driedmixture weighs 63.7 grams and contains 5 percent by weight rare earthoxide mixture, 5 percent by weight chromia, 5 percent by weight bariaand 85 percent by weight alumina. The mixture is crushed to powder and aportion is calcined at 1200°C. for four hours. The surface area of thecalcined powder is 36.4 m² /g.

EXAMPLE V

A composite is prepared containing ceria-chromia-baria-alumina 7.57grams of cerium nitrate, and 3.95 grams of CrO₃ (chromic acid) aredissolved in 70 ml. of H₂ O. 57 grams of admixture A, i.e.,baria-alumina admixture from EXAMPLE IV, is added to the solution withconstant agitation for 10 minutes. The slurry is then evaporated todryness with heat and agitation, transferred to a drying oven at 110°C.,and then dried overnight. The dried solids weighing 66 grams are groundto a powder. 5 grams of the composite containing 5% ceria, 5% chromia,5% baria, and 85% alumina is then calcined for 4 hours at 1200°C. It isfound that the surface area after such calcination is 36.5 m² /g.

EXAMPLE VI

A neodymia-chromia-baria-alumina composite is prepared. 7.82 grams ofneodymium nitrate and 3.95 grams of CrO₃ (chromic acid) are dissolved inwater and diluted to 70 ml. 57 grams of admixture A (i.e., thebarium-aluminum from EXAMPLE IV) are added to the solution and theslurry is mixed for 5 minutes. The slurry is then evaporated to drynesswith heat and agitation, transferred to a drying oven at 110°C. anddried overnight. The dried solids are ground to a powder. A portion ofthe powder is then calcined at 1200°C. for 4 hours. It is found that thesurface area after such calcination is 20.9 m² /g.

EXAMPLE VII

A composite is prepared by coprecipitation. The composition is 5 percentof ceria, 5 percent chromia, 5 percent strontia, and 85 percent alumina.156 grams of aluminum nitrate, 3.15 grams of cerium nitrate, 2.34 gramsof strontium nitrate, and 6.6 grams of chromium nitrate are dissolved inseries in one liter of water and the solution transferred to a droppingfunnel. A second solution is prepared containing 400 ml. of ammoniumhydroxide (28.3% NH₃) and 1600 ml. water and transferred to a droppingfunnel. 2000 ml. of water is added to a 6 liter breaker with vigorousmechanical stirring. The nitrate solution is then added at roomtemperature to the water in the beaker over a period of 30 minutes. Theammonia solution is added concurrently with the nitrate solution at sucha rate as to keep the pH of the slurry in the beaker at 9.0. Stirring iscontinued for 15 minutes after the coprecipitation is complete. Theslurry is allowed to stand overnight and then filtered and re-slurriedin 2 liters of water. The second slurry is filtered, excess waterremoved, and dried for four days at room temperature. The filter cake ishand ground to a powder, dried for 1 day at room temperature, andovernight at 110°C. The surface area is good after calcination at1200°C. for 2 hours.

EXAMPLE VIII

A stabilized Nd₂ O₃, Cr₂ O₃, SiO₂, and Al₂ O₃ composite is prepared.7.82 grams of neodymium nitrate, 3.95 grams of CrO₃ (chromic acid) areadded to a sol containing 10.00 grams of Ludox LS in 40 ml. of water.The components are dissolved and enough water added to form a totalvolume of 80.3 ml. 51 grams of activated Al₂ O₃ powder is stirred intothe solution with constant agitation for 10 minutes. The total solutionis then evaporated to dryness under heat and with agitation, transferredto a drying oven at 110°C., and dried overnight. The dried solids arethen crushed to powder. Five grams of the composite containing 5%neodymia, 5% chromia, 5% silica, and 85% alumina is then tested forretention of surface area by calcining at 1200°C. for 4 hours. It isfound that the surface area after such calcination is 34.4 m² /g.

EXAMPLE IX

A stabilized Nd₂ O₃, Cr₂ O₃, SnO, and Al₂ O₃ composite is prepared. 7.82grams of neodymium nitrate and 3.95 grams of CrO₃ (chromic acid) isadded to a solution composed of 5.03 grams of stannous chloridedissolved in 20 ml. of water and 1 ml. conc. HCl. The components aredissolved and enough water added to form a total volume of 80.3 ml. 51grams on activated Al₂ O₃ powder is stirred into the solution withconstant agitation for 10 minutes. The total solution is then evaporatedto dryness under heat and with agitation, transferred to a drying ovenat 110°C. and dried overnight. The dried solids are then crushed topowder. Five grams of the composite containing 5% neodymia, 5% chromia,5% SmO and 85% alumina is then tested for retention of surface area bycalcining at 1200°C. for 4 hours. It is found that the surface areaafter such calcination is 45.9 m² /g.

In the practice of this invention the catalytic compositions areparticularly useful when employed with the high temperature oxidation ofcarbonaceous fuels. For example, they may be used advantageously in amethod employing a catalytically-supported thermal combustion ofcarbonaceous fuel, as more fully described in co-pending applicationSer. No. 358,411, filed May 8, 1973, of W. C. Pfefferle, assigned to theassignee hereof and which application is incorporated by referenceherein. This method includes the essentially adiabatic combustion of atleast a portion of a carbonaceous fuel admixed with air in the presenceof a catalytic composition of this invention at an operating temperaturesubstantially above the instantaneous auto-ignition temperature of thefuel-air admixture but below a temperature that would result in anysubstantial formation of oxides of nitrogen.

Flammable mixtures of most fuels with air are normally such as to burnat relatively high temperatures, i.e., about 3300°F. and above, whichinherently results in the formation of substantial amounts of nitrogenoxides or NO_(x). However, little or no NO_(x) is formed in a systemwhich burns the fuel catalytically at relatively low temperatures.

For a true catalytic oxidation reaction, one can plot temperatureagainst rate of reaction. For any given catalyst and set of reactionconditions, as the temperature is initially increased, the reaction rateis also increased. This rate of increase is exponential withtemperature. As the temperature is raised further, theh reaction ratethen passes through a transition zone where the limiting parametersdetermining reaction rate shift from catalytic to mass transfer. Whenthe catalytic rate increases so such an extend that the reactants cannotbe transferred to the catalytic surface fast enough to keep up with thecatalytic reaction rate, the reaction shifts to mass transfer control,and the observed reaction rate changes much less with furthertemperature increases. The reaction is then said to be mass transferlimited. In mass transfer controlled catalytic reactions, one cannotdistinguish between a more active catalyst and a less active catalystbecause the intrinsic catalyst activity is not determinative of the rateof reaction. Regardless of any increase in catalytic activity above thatrequired for mass transfer control, a greater catalytic conversion ratecannot be achieved for the same set of conditions.

It has been discovered that it is possible to achieve essentiallyadiabatic combustion in the presence of a catalyst at a reaction ratemany times greater than the mass transfer limited rate. That is,catalytically-supported, thermal combustion surmounts the mass transferlimitation. If the operating temperature of the catalyst is increasedsubstantially into the mass transfer limited region, the reaction rateagain begins to increase exponentially with temperature. This is anapparent contradiction of catalytic technology and the laws of masstransfer kinetics. The phenomena may be explained by the fact that thecatalyst surface and the gas layer near the catalyst surface are above atemperature at which thermal combustion occurs at a rate higher than thecatalytic rate, and the temperature of the catalyst surface is above theinstantaneous autoignition temperature of the fuel-air admixture(defined hereinbelow). The fuel molecules entering this layerspontaneously burn without transport to the catalyst surface. Ascombustion progresses, it is believed that the layer becomes deeper. Thetotal gas is ultimately raised to a temperature at which thermalreactions occur in the entire gas stream rather than only near thesurface of the catalyst. At this point, the thermal reactions continueeven without further contact of the gas with the catalyst as the gaspasses through the combustion zone.

The term "instantaneous auto-ignition temperature" for a fuel-airadmixture as used herein and in the appended claims is defined to meansthat the ignition lag of the fuel-air mixture entering the catalyst isnegligible relative to the residence time in the combustion zone of themixture undergoing combustion.

This method can employ an amount of fuel equivalent in heating value ofabout 300-1000 pounds of propane per hour per cubic foot of catalyst.There is no necessity of maintaining fuel-to-air ratios in the flammablerange, and consequently loss of combustion (flame-out) due to variationsin the fuel-to-air ratio is not as serious a problem as it is inconventional combustors.

The adiabatic flame temperature of fuel-air admixtures at any set ofconditions (e.g., initial temperature and, to a lesser extent, pressure)is established by the ratio of fuel to air. The admixtures utilized aregenerally within the inflammable range or are fuel-lean outside of theinflammable range, but there may be instances of a fuel-air admixturehaving no clearly defined inflammable range but nevertheless having atheoretical adiabatic flame temperature within the operating conditionsof the invention. The proportions of the fuel and air charged to thecombustion zone are typically such that there is a stoichiometric excessof oxygen based on complete conversion of the fuel to carbon dioxide andwater. Preferably, the free oxygen content is at least about 1.5 timesthe stoichiometric amount needed for complete combustion of the fuel.Although the method is described with particularity to air as thenon-fuel component, it is well understood that oxygen is the requiredelement to support proper combustion. Where desired, the oxygen contentof the non-fuel component can be varied and the term "air" as usedherein refers to the non-fuel components of the admixtures. The fuel-airadmixture fed to the combustion zone may have as low as 10 percent freeoxygen by volume or less, which may occur, for example, upon utilizationas a source of oxygen of a waste stream wherein a portion of this oxygenhas been reacted. In turbine operations, the weight ratio of air to fuelcharged to the combustion system is often above about 30:1 and someturbines are designed for air-to-fuel ratios of up to about 200 ormore:1.

The carbonaceous fuels may be gaseous or liquid at normal temperatureand pressure. Suitable hydrocarbon fuels may include, for example, lowmolecular weight aliphatic hydrocarbons such as methane, ethane,propane, butane, pentane; gasoline; aromatic hydrocarbons such asbenzene, toluene, ethylbenzene, xylene; naphtha; diesel fuel; jet fuel;other middle distillate fuels; hydrotreated heavier fuels; and the like.Among the other useful carbonaceous fuels are alcohols such as methanol,ethanol, isopropanol; ethers such as diethylether and aromatic etherssuch as ethylphenyl ether; and carbon monoxide. In burning diluted fuelscontaining inerts, for example, low BTU coal gas, fuel-air admixtureswith adiabatic flame temperatures within the range specified herein maybe either fuel rich or fuel lean. Where fuel rich mixtures are utilized,additional air or fuel-air admixture may be added to the catalyst zoneeffluent to provide an overall excess of air for complete combustion offuel components to carbon dioxide and water. As stated previously,thermal reactions continue beyond the catalyst zone, provided theeffluent temperature is substantially above the instantaneousauto-ignition temperature.

The fuel-air admixture is generally passed to the catalyst in thecombustion zone at a gas velocity prior to or at the inlet to thecatalyst in excess of the maximum flame propagating velocity. This maybe accomplished by increasing the air flow or by proper design of theinlet to a combustion chamber, e.g., restricting the size of theorifice. This avoids flashback that causes the formation of NO_(x).Preferably, this velocity is maintained adjacent to the catalyst inlet.Suitable linear gas velocities are usually above about 3 feet persecond, but it should be understood that considerably higher velocitiesmay be required depending pon such factors as temperature, pressure, andcomposition. At least a significant portion of the combustion occurs inthe catalytic zone and may be essentially flameless.

The carbonaceous fuel, which when burned with a stoichiometric amount ofair (atmosphere composition) at the combustion inlet temperature usuallyhas an adiabatic flame temperature of at least about 3300°F., iscombusted essentially adiabatically in the catalyst zone. Although theinstantaneous auto-igntion temperature of a typical fuel may be belowabout 2000°F., stable, adiabatic combustion of the fuel below about3300°F. is extremely difficult to achieve in practical primarycombustion systems. It is for this reason that even with gas turbineslimited to operating temperatures of 2000°F., the primary combustion istypically at temperatures in excess of 4000°F. As stated above,combustion in this method is characterized by using a fuel-airadmixture, having an adiabatic flame temperture substantially above theinstantaneous auto-ignition temperature of the admixture but below atemperature that would result in any substantial formation of NO_(x).The limits of this adiabatic flame temperature are governed largely byresidence time and pressure. Generally, adiabatic flame temperatures ofthe admixtures are in the range of about 1700°F. to 3200°F., andpreferably are about 2000°F. to 3000°F. Operating at a temperature muchin excess of 3200°F. results in the significant formation of NO_(x) evenat short contact times; this derogates from the advantages of thisinvention vis-a-vis a conventional thermal system. A higher temperaturewithin the defined range is desirable, however, because the system willrequire less catalyst and thermal reactions are an order of magnitude ormore faster, but the adiabatic flame temperature employed can depend onsuch factors as the desired composition of the effluent and the overalldesign of the system. It thus will be observed that a fuel which wouldordinarily burn at such a high temperature as to form NO_(x) issuccessfully combusted within the defined temperature range withoutsignificant formation of NO_(x).

The catalyst used in this method generally operates at a temperatureapproximating the theoretical adiabatic flame temperature of thefuel-air admixture charged to the combustion zone. The entire catalystmay not be at these temperatures, but preferably a major portion oressentially all, of the catalyst surface is at such operatingtemperatures. These temperatures are usually in the range of about1700-3200°F., preferably about 2000°F. to about 3000°F. The temperatureof the catalyst zone is controlled by controlling the combustion of thefuel-air admixture, i.e., adiabatic flame temperature, as well as theuniformity of the mixture. Relatively higher energy fuels can be admixedwith larger amounts of air in order to maintain the desired temperaturein a combustion zone. At the higher end of the temperature range,shorter residence times of the gas in the combustion zone appear to bedesirable in order to lessen the chance of forming NO_(x).

The residence time is governed largely by temperature, pressure, andspace throughput; and generally is measured in milliseconds. Theresidence time of the gases in the catalytic combustion zone and anysubsequent thermal combustion zone may be below about 0.1 second,preferably below about 0.05 second. The gas space velocity may often be,for example, in the range of about 0.5 to 10 or more million cubic feetof total gas (standard temperature and pressure) per cubic foot ofvolume of total combustion zone per hour. For a stationary turbineburning diesel fuel, typical residence times could be about 30milliseconds or less; whereas in an automotive turbine engine burninggasoline, the typical residence time may be about 5 milliseconds orless. The total residence time in the combustion system should besufficient to provide essentially complete combustion of the fuel, butnot so long as to result in the formation of NO_(x).

A method employing the catalyst of the present invention is exemplifiedin a series of runs in which the fuel is essentially completelycombusted, and a low emissions effluent produced. The combustion systemcomprises a source of preheated air supplied under pressure. A portionof the air is passed through a pipe to the combustion zone, and theremainder is used to cool and dilute the combuston effluent. Unleadedgasoline fuel is atomized into the air passing to the combustion zonecountercurrent to the air flow to insure intimate mixing.

The catalyst used is of the monolithic, honeycomb-type having a nominal6-inch diameter and is disposed within a metal housing as two separatepieces each having parallel flow channels 21/4 inches in lengthextending therethrough. There is a small space of about 1/4 inch betweenthese pieces. Both pieces of catalyst have approximately 100 flowchannels per square inch of cross-section with the walls of the channelshaving a thickness of 10 mils. The catalysts have similar compositionsand are composed of a zircon mullite honeycomb support which carries acomposite coating of alumina, chromia, neodymia, and strontia containingpalladium.

Provision is made for contacting the fuel mixed with a portion of thetotal air stream with the catalyst. That portion of the total air streamnot mixed with the fuel is added to the combustion effluent immediatelyupon its exit from the catalyst zone. This dilution or secondary aircools the combustion effluent and samples of the mixture are taken foranalysis. Thermocouples are located adjacent the initial catalyst inletand at the sampling position to detect the temperatures of theselocations.

The catalysts are brought to reaction temperature by contact withpreheated air, and subsequent contact with the air-fuel mixture whichcauses combustion and raised the catalyst temperature further. Theresults obtained using this system during two periods of operation inaccordance with the present invention exhibit low pollutant emission.

The catalyst of this invention can also be used for selected oxidationreactions at lower temperatures. In a typical oxidation they can beemployed to promote the reaction of various chemical feedstocks bycontacting the feedstock or compound with the catalyst in the presenceof free oxygen preferably molecular oxygen. Although some oxidationreactions may occur at relatively low temperatures, many are conductedat elevated temperatures of about 150°C. to 900°C., and generally, thesereactions occur with the feedstock in the vapor base. The feedsgenerally are materials which are subject to oxidation and containcarbon, and may, therefore, be termed carbonaceous, whether they areorganic or inorganic in character. The catalysts of this invention areparticularly useful in promoting the oxidation of hydrocarbons,oxygen-containing organic components, for example, aldehydes, organicacids, and other intermediate products of combustion, such as carbonmonoxide, and the like. These materials are frequently present inexhaust gases from the combustion of carbonaceous fuels, and thus thecatalysts of the present invention are particularly useful in promotingthe oxidation of such materials thereby purifying the exhaust gases.Such oxidation can be accomplished by contacting the gas stream with thecatalyst and molecular or free oxygen. The oxygen may be present in thegas stream as part of the effluent, or may be added as air or in someother desired form having a greater or lesser oxygen concentration. Theproducts from such oxidation contain a greater weight ratio of oxygen tocarbon than in the material subjected to oxidation and in the case ofexhaust purification these final oxidation products are much lessharmful than the partially oxidized materials. Many such reactionsystems are known in the art.

What is claimed is:
 1. A method for the combustion of carbonaceous fuelcomprising: forming an intimate admixture of said fuel and air;contacting said fuel air admixture with an oxidation catalyst at atemperature sufficient to combust said admixture, said catalyst having asurface area of at least 20 m² /g after calcination for two hours at atemperature of 1200°C., said catalyst consisting essentially of (a) acatalystically-active calcined composite of alumina, a rare earth metaloxide, and a mixture of two metal oxide components wherein the firstcomponent is selected from the group consisting of an oxide of Cr, W,and mixtures thereof and the second component is selected from the groupconsisting of an oxide of calcium, strontium, barium, silicon, tin, andmixtures thereof, and (b) a catalytically-effective amount of platinumgroup metal incorporated therein after calcination of said composite ata temperature of at least 500°C.
 2. A method as defined in claim 1,wherein said combustion is catalytically-supported thermal combustionforming an effluent of high thermal energy said fuel being in vaporousform and intimately admixed with air; said combustion being underessentially adiabatic conditions and being characterized by saidfuel-air admixture having an adiabatic flame temperature such that uponcontact with said catalyst the operating temperature of said catalyst issubstantially above the instantaneous auto-ignition temperature of saidfuel-air admixture but below a temperature that would result in anysubstantial formation of oxides of nitrogen comprising: contacting saidfuel-air admixture with an oxidation catalyst having a surface area ofat least 20 m² /g after calcination for two hours at a temperature of1200°C., said catalyst consisting essentially of (a) acatalytically-active calcined composite of alumina, a rare earth metaloxide, and a mixture of two metal oxide components wherein the firstcomponent is selected from the group consisting of an oxide of Cr, W,and admixtures thereof, and the second component is selected from thegroup consisting of an oxide of calcium, strontium, barium, silicon,tin, and mixtures thereof, and (b) a catalytically-effective amount ofplatinum group metal added thereto after calcination of said compositeat a temperature of at least 500°C.
 3. A method as defined in claim 2further comprising depositing said composite on a relatively inertsubstrate to form a coating thereon prior to said platinum group metaladdition.
 4. A method as defined in claim 3 wherein said substrate is ahoneycomb.